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This study examines the effects of a reduction in the phosphorus input (in comparison to field levels) on natural coastal phytoplankton communities. Experiments were conducted in four land-based mesocosms consisting of 3 m(3) black polyethylene tanks. Light climate and mixing regime reproduced conditions in Dutch coastal waters, temperature was maintained within 2 degrees C of the ambient seawater temperature. Phytoplankton species succession and primary production were studied in two mesocosm experiments of 5 and 4 weeks in April and July 1993. In both experiments, inorganic nutrients were continuously added to four mesocosms. N (nitrogen) and Si (silicon) inputs were identical for the four mesocosms and resembled the inputs to the Dutch coastal zone. The phosphorus (P) input established a N/P ratio gradient among the mesocosms: 16, 32, 64 and 128. The first experiment was carried out during the spring diatom bloom. The initial diatom dominance was maintained in the mesocosms with reduced phosphorus loads. In the mesocosm with the highest P input, flagellates developed to the detriment of the diatoms at the end of the experiment in parallel with the establishment of a silicate limitation. The second experiment coincided with the spring/summer phytoplankton bloom dominated by the colony of Phaeocystis sp. Diatoms outcompeted Phaeocystis sp. in the mesocosms with reduced phosphorus inputs (N/P = 32, 64, 128). In both experiments, maximum primary production rates were observed in the mesocosm with the highest P input and decreased with the phosphorus gradient. It was concluded that a reduction of the phosphorus loads to eutrophicated coastal waters determined a proportional reduction of the primary production and stimulated the dominance of diatoms over flagellates. [KEYWORDS: primary production; species succession; zooplankton grazing; mesocosm North-sea; plankton communities; eutrophication; competition; nutrient]